I looked up C02 prices online and came up with some price comparisons. These are average prices of C02 for carbonating a 2L bottle of water:
-CO2 bought to fill 20lb tank in NYC: $0.20
Initial investment: $150–200
-Soda Stream, 3rd party cartridges on Amazon: $0.50
Initial investment: $50–$80
-Generic Seltzer bought at supermarket: $0.70–$1.20
Initial Investment: $0
-iSi siphon (2 chargers/liter), 3rd party chargers: $1.76
Initial investment: $100–150
Seems like from an economics point of view, the whole enterprise mostly makes sense if you want to carbonate stuff besides water, or if you consume a LOT of seltzer.
If you're in the I-want-bubbles-in-all-kinds-of-stuff camp, one reason to consider the big tank, under-counter, DIY approach is that you can also use nitrous oxide. Dave Arnold talks about this in some detail. The idea is that N2O gives a completely different texture (softer bubbles) and flavor (sweet, not sour), and so being able to mix both kinds of gas with your regulator gives you fantastic control. He shows a picture of his under-counter setup, but declines to give instructions, since technically, N2O is a controlled substance (20lbs of whip-its, anyone?). But he invites you to decipher the picture, and assures you that you can indeed buy pharmaceutical grade N2O in bulk from welding shops, if you ask the right questions (you don't want to use industrial grades in anything you eat or drink).
